Macular edema is an important cause of visual loss and legal blindness in patients with diabetic retinopathy. In the Early Treatment Diabetic Retinopathy Study (ETDRS) [1], focal laser photocoagulation was demonstrated to reduce the risk of moderate vision loss in diabetic patients with an entity termed clinically significant macular edema (CSME). As optical coherence tomography (OCT) was not available at the time of the EDTRS study, CSME was defined based on biomicroscopic observations by the examining physician. Three definitions of CSME were adopted by the ETDRS investigators: (1) presence of any retinal thickening within 500 μm of the foveal center, (2) lipid exudates within 500 μm of the foveal center with adjacent thickening, and (3) an area of thickening >1 Macular Photocoagulation Study disc area (DA; 1 DA≅1.767 mm2) within 1 disk diameter (1.5 mm) of the foveal center. To corroborate and standardize the clinical assessment, the ETDRS Fundus Photographic Reading Center reviewed color stereoscopic photographs for a number of imaging end points, including the presence and extent of macular edema. Because accurate methods of quantifying axial retinal thickening were not available at that time, macular edema extent, as determined by biomicroscopic examination or inspection of stereoscopic photographs, was based only on the area of thickening and not on the magnitude of the axial thickness. Variations in the amount of stereopsis present in paired stereo photographs or in the threshold for thickening adopted by the observer may further complicate the accurate and reproducible detection of areas of edema. Thus, there is potential for considerable variability and possible lack of sensitivity in prior art methods for identifying macular edema that were used in previous clinical studies.
Furthermore, the lack of sensitivity of the clinical examination for detection of mild edema has been demonstrated by a number of investigators, including Brown et al. [2], who observed that, for eyes with a foveal center thickness between 201 and 300 μm (200 defined as the upper limit of normal), only 14% were noted to have foveal edema by contact lens biomicroscopy. They coined the term subclinical foveal edema to describe such cases. However, as these cases of subclinical edema were presumably not recognized in the ETDRS, the rationale for treating these lesions is presently uncertain, although it may change as data from ongoing clinical trials incorporating OCT imaging become available.
Although the system proposed by Brown et al. is useful for identification of foveal edema, it is not likely to identify cases of nonfoveal CSME. As OCT has become an integral part of clinical trials and clinical practice, a system for detection of nonfoveal CSME would be valuable. Unfortunately, the commonly used macular scanning patterns on the Stratus OCT machine (Carl Zeiss Meditec, Inc., Dublin, Calif.) consist of radial lines that provide a high density of points near the fovea, but a relatively sparse pattern in more peripheral zones. This requires considerable interpolation to construct a thickness map.
Therefore, there still exists a need for better methods to identify macular edema.